Introduction
Solar water heating
is a technology that every homeowner can use to save dollars on utility
bills. Questions most frequently addressed to the Florida Solar Energy
Center (FSEC) about solar water heating are answered below. Please note
that many replies are general and more explanation may be required for
those with individual questions. For additional information, contact the
FSEC Public Information Office, call a solar dealer, or consult the references
listed.
Table of Contents:
Q: What is a solar water
heater?
A solar water heater uses the sun's energy rather than electricity or
gas to heat water, thus reducing your monthly utility bill. When installed
properly, solar water heaters are more economical over the life of the
system than heating water with electricity, dedicated heat pumps, heat
recovery units or propane.
In Florida, three types of solar systems are used: pumped, integral collector
storage (ICS), and thermo-siphon. The direct circulation system (see
Figure 1, page 2) circulates potable water from the water storage
tank through one or more collectors and back into the tank. The solar
collector is the main component of the solar system. It is usually a metal
box with insulation and a black absorber plate that collects solar radiation
and heats the water. The circulating pump is regulated by either an electronic
controller, a common appliance timer, or a photovoltaic (PV) panel.
In integral collector storage systems, the solar water storage system
is built into the collector. The potable water in the collector unit is
heated by the sun and delivered by city or well water pressure to an auxiliary
tank (which contains non-solar back-up heating) or directly to the point
of use.
A thermosiphon solar water heating system has a tank mounted above the
collector (normally on the roof) to provide a natural gravity flow of
water. Hot water rises through piping in the collector, which is mounted
below the tank; heavier cold water sinks to the lowest point in the system
(the collector), displacing the lighter hot water which rises to the tank.
The ICS and thermosiphon systems are simple since they use no pumps or
controllers and water always flows through the collector. <Top>
Q: How does a direct circulation
solar system work?
As sunshine strikes the collector, the water inside it is heated. If
the circulating pump is regulated by a PV panel, the pump starts turning
as the PV panel is activated by the same sunshine. This direct current
(DC) motor pump moves water from the tank through the collector and back
to the tank. As the sun's intensity changes throughout the day, the circulating
pump also changes its speed accordingly. By the end of the day, the water
in the tank has been circulated many times through the collector and has
been heated to usable hot water temperatures.
If the circulating pump is regulated by an electronic differential controller,
a sensor at the outlet of the collector and a sensor at the bottom of
the tank (Figure 1) activate the circulating pump when the water
in the collector is about 15-200 F warmer than the water at
the bottom of the tank. The pump then circulates water from the collector
and the tank. This process continues as long as the water temperature
at the collector outlet is about 50 F higher than that in the
bottom of the tank. If the temperature difference decreases further, the
controller automatically shuts off the pump.
Common appliance timers also may control system operation. The timer
is set to operate during a period of the day when solar radiation is available
to heat the potable water. It is important that the timers used in these
systems incorporate battery back-up in the event of power failures. In
order to avoid loss of energy from the tank during overcast days, the
collector feed and return lines are both connected at the bottom of the
storage tank. During normal operation, natural stratification allows the
warmer water to rise to the top part of the tank.
During periods of insufficient sunshine or high hot water demand, a backup
electrical element in the storage tank heats the water. The check valve
prevents heat loss when the circulating pump is off. The circulating pump
consumes only a small amount of electricity — around $5 to $10 worth per
year, or in the case of PV — none. <Top>
Figure 1. A solar water heating systemQ: What size solar system
do I need?
For Florida residences with a dishwasher and an automatic clothes washer,
a rough rule of thumb is 10-square-feet of collector area, per person,
and 20 gallons of water storage, per person. Thus, for a family of four,
40-square-feet of collector and a total tank storage of 80 gallons are
typical. It must be noted that ICS and thermosiphon systems will include
two tanks, the solar system tank as well as the auxiliary tank.
Families can also purchase a solar-assisted system that uses a 20-to
26-square-foot collector that is retrofitted to their existing 40-or 52-gallon
water heater. Due to the reduced storage tank capacity and smaller collector
size, these systems will only provide 50-60% of the hot water used. The
installed price will of course be lower since the existing tank and smaller
collector are used. <Top>
Q: How much do I save?
Your monthly savings will depend on the amount of hot water you use,
storage tank size and type and price of fuel used for backup water heating.
For a family of four, the typical hot water usage is 70- gallons per day,
thus using 3990-kilowatt-hours per year to heat the water electrically,
or $399 per year worth of electricity at 10¢ per kilowatt hour.
A solar water heater should save between 50% and 85% of the hot water
portion of the monthly utility bill, or $200 to $300 per year
for a family of four, if the backup element is kept at 122 F.
A solar water heater can save even more if you turn off the backup and
rely solely on the sun for your hot water. During summer months, when
hot water demands are lower and the sun shines longer, most solar owners
turn off the backup element circuit breaker, or switch. As electricity
and other fuel prices go up, solar savings will increase accordingly.
An FSEC fact sheet, Solar Hot Water Energy and Cost Savings for Typical
Florida Residential Installation, provides an overview of the potential
savings from various solar systems available in Florida. <Top>
Q: Do I have to change my
habits to use a solar water heater?
No. Solar water heaters are always installed with a backup heating system
in the storage tank to ensure that hot water is available at all times.
However, to maximize solar utilization and your savings, you should attempt
to use the most hot water in the late morning and early afternoon when
the solar system is operating at its peak due to the available sunshine.
Also, your solar system will be more effective if your use of hot water
is spread more evenly over the week. For example, if you use hot water
for laundry, instead of washing seven loads of clothes in a single day,
wash one load each day. This will reduce the amount of backup energy required
for your solar system. <Top>
Q: How much does a solar
water heater cost?
In Florida, an installed solar water heating sys-tem can cost anywhere
from $1,500 to $3,500. Why the large range? System cost depends upon certain
variables, such as the following:
Size of family to be served
Size and type of solar system
Type of financing available
Type of roof on which the collector is to be installed
Amount of possible utility rebate incentive
Building code requirements
Professional versus do-it-yourself installation
Solar water heating is economically competitive with electrical and propane
heating. It is also competitive with dedicated heat pumps and heat recovery
units, depending upon the individual situation. <Top>
Q: Does government offer
any financial incentives?
There are currently no federal tax credits available. In 1996, the Florida
Legislature passed a state sales tax exemption for solar systems. Individuals
should call their local utility company (electric or gas) about utility
company rebates or incentives for solar water heating. <Top>
Q: How do I finance a solar
water heating system?
The best way to finance a solar energy system is to include it as part
of your home mortgage. In long-term loans of 20 years or more, the monthly
solar savings will normally be greater than the monthly mortgage payments
for the solar system.
If you are buying an existing home with a new mortgage, you may be able
to include a new solar water heater in the mortgage. Check with your local
lending office. Also, many Florida lending institutions offer short-term
loans for solar systems, based upon the applicant's personal credit rating.
Some solar companies also offer system financing. <Top>
Q: What kind of a solar
collector should I buy?
There are basically two types of solar collectors available for solar
water heating. The first is referred to as a flat-plate glazed solar collector.
The second is the integral collector storage (ICS) collector (Figure
2). Both collectors are mounted in a fixed position throughout the
year and do not track the sun as do some collectors that are used primarily
for high-temperature applications.
A basic flat-plate collector consists of a metal enclosure, an absorber
plate and flow tubes, insulation, and a glass covering. Flat-plate collectors
can be 2 to 4 feet wide, 5 to 12 feet long, and 4 inches thick. The absorber
takes in the sun's energy, which is then transferred to the water flowing
through the tubes attached to the absorber.
An ICS collector consists of large tubes, often 4 inches in diameter,
in which potable water is both heated and stored in a combined heat storage
and collection unit. As with the flat-plate collector, the ICS unit also
consists of a metal enclosure, insulation, and a glass covering. ICS units
are available in sizes ranging from 30 to 50 gallons, and can range from
4 feet in width to 8 feet in length to 10 inches in depth.
To reduce heat losses, all flat-plate and ICS collectors generally have
insulation behind the absorber plate and a glass cover on the front, facing
the sun. The best cover material is tempered glass of low iron content.
Some edge insulation inside the enclosure box is also necessary.
The absorber plate is made of copper and is coated with a black chrome
or nickel material called a selective surface. This surface greatly enhances
the collector's ability to capture and retain solar energy. Some manufacturers
also use black paint as an absorber coating.
The Florida Solar Energy Center conducts a state-mandated program of
collector testing and certification. All collectors now sold in Florida
should bear the Center's certification label.
A document entitled Thermal Performance Ratings, available from
the FSEC Public Information office, contains the performance rating for
each solar collector certified by the Center. The document also provides
an approximate efficiency-per-dollar comparison method you can use as
a guide for rejecting inefficient or overly expensive collectors. <Top>
Q: Since there are several
different types of solar water heating systems, how can I compare their
performance?
FSEC conducts a state-mandated standards program for solar domestic hot
water and solar swimming pool heating systems. The same solar collector
can perform differently when installed in different system types. In addition,
the other components (tank, pump, controller, etc.) selected for a particular
system can have a large effect on the overall performance of a solar system.
To allow comparison of complete systems, the "Florida Energy Factor" was
devised to rate the performance of all solar system types. This factor
is similar to the rating given to electric and gas water heaters.
The higher the energy factor, the more a solar water heater will save.
The FSEC Public Information Office can supply energy factors for Florida
solar systems, as can your solar vendor. <Top>
Q: What is the proper orientation
of the solar collector?
Collectors should be mounted on an unshaded area of a south-facing roof.
They can face up to 45 degrees east or west of south without a significant
decrease in performance.
For all shingle and tile roofs in Florida that generally have pitches
greater than 3 in 12 (i.e., 14 degrees), collectors should be mounted
parallel to the roof. Collectors mounted in this manner are more aesthetically
pleasing. However, for flat or very low-sloping roofs, collectors should
be tilted at an angle (to the horizontal) that is approximately equal
in degrees to the local latitude. Florida latitudes range between 25 degrees
(in the Florida Keys) to 31 degrees (northern border). Since the sun is
lower on the horizon during the winter months, tilting the collector at
an angle up to 15 degrees greater than latitude will increase winter performance,
which is desirable in most cases. <Top>
Q: Where can I buy a solar
system?
Firms installing solar water heaters are listed in the telephone book
or advertised in local papers. A Florida Solar Industry Directory
and additional consumer information may be ordered from FSEC's Public
Information Office. <Top>
Q: How do I decide which
solar water heating dealer to contact?
Compare price, efficiency, service options, dealer reputation and warranties.
Ask the solar dealer for a list of previous customers, and talk to them.
Ask the owners about performance. Does the system provide sufficient hot
water? Was it affected by last winter's freeze? Inquire whether service
has been necessary and if it was satisfactory. Ask about the warranty
offered and read it carefully. A reputable dealer/installer will normally
repair or replace a system component that malfunctions in the first year
if the problem is caused by faulty materials, workmanship or installation.
The quality of installation can radically affect the reliable operation
of a solar system. Installers should be licensed to install solar water
heating systems. Installers should have either a state Solar Contractor
or a Residential Solar Water Heating Specialty license or possess a separate
license such as plumbing or air conditioning, which authorizes them to
install solar residential hot water systems. Some local governments also
have established a special solar contractor license.
In addition, many solar installers in the state belong to the Florida
Solar Industries Association and abide by its Code of Ethics. As with
other trades, your local Better Business Bureau can reveal any service
or installation complaints against a vendor. <Top>
Q: How do I protect my
solar system from freezing weather?
Freeze protection is an absolute must for any solar system because a
single freeze can destroy a collector. Even in Miami, collector tubes
have frozen and burst during hard winter freezes.
In order to avoid problems, Florida Solar Energy Center standards require
that all solar systems be designed and installed with two freeze protection
methods. The strategies and any instructions for the homeowner should
be posted on a label affixed to a prominent location on the solar system,
(usually the storage tank). There are a number of options available to
prevent freeze damage to solar systems.
In the south and central Florida areas, flush-type freeze prevention
valves are being used. These valves open when the air temperature approaches
freezing. Water pressure forces water from the storage tank through the
collectors and out through the valve. The valve closes when the water
leaving the valve reaches 500 F. This process prevents the
water in the collector tubes from freezing and rupturing the tubes. Though
water is purged from the system and lost, only a few gallons are needed
to protect the collector from freezing.
Another method of freeze protection is achieved by water recirculation.
When the temperature drops below 400 F, a collector freeze
sensor activates the pump to circulate warm water through the collectors.
A second level of protection in this system, in the event of power failures,
is provided by drain valves that manually drain the collector. Manual
protection alone may be adequate for conscientious homeowners in southern
Florida areas where hard freezes are rare. It is best to review the manual
draindown instructions provided on the freeze information label or call
your installer prior to manually draining your system.
ICS systems incorporate thermal mass freeze protection. The large size
of the tanks in the ICS units requires a much longer and more severe period
of freeze temperatures before all the water in the tubes freezes. Florida
freezes are generally not severe enough to turn all the water in ICS tanks
into ice. Nevertheless, piping on the roof and in attics must also be
protected. ICS systems in central and north Florida also incorporate a
freeze prevention valve as a secondary freeze protection mechanism.
In north Florida, an indirect system is often used (Figure 3).
An antifreeze solution, similar to that used in automobiles, is circulated
through the collector. A heat exchanger between the collector and storage
tank transfers heat from the antifreeze to the potable water. Although
this system is usually more expensive and less efficient than direct systems
(in which the water used for bathing, etc., is the same water circulated
through the collectors), it provides better automatic freeze protection
in colder climates.
In some cases, thermosiphon systems also incorporate a heat exchanger
in their design, which makes the thermosiphon design quite suitable for
colder climates.
Two other types of freeze protection methods —automatic drain-down and
drain-back systems — are used extensively in the northern United States
and are also suitable for north and central Florida. Drain-down systems
normally use electrically operated valves that automatically drain the
water from the collector during freezing temperatures. In drain-back systems,
the collector is automatically drained whenever the circulating pump stops.
With careful design and installation, both drain-down and drain-back systems
can be fail-safe and cost effective. <Top>
Figure 3. Indirect pumped system using antifreeze solution Q: Are solar water heating
systems reliable?
Properly designed and installed systems with glass-covered collectors
should perform well for more than 20 years. Controllers, like other electronic
devices, may require servicing during the life of the system, and the
pump and hot water tank may have to be replaced after 10 years. Since
conventional water heaters have the same expected lifetime, water tank
replacement costs are not regarded as unique to solar energy systems.
Normal maintenance consists of checking pipe insulation, roof penetrations
and collector mounting, pump operation and tank flushing. The latter is
also recommended for conventional water heating systems, as is periodic
replacement of the water heater sacrificial anode rod.
Many installation firms provide yearly maintenance check-ups of their
solar systems similar to annual air-conditioning system maintenance programs.
These can be beneficial in extending the life of the system and ensuring
optimum performance.
It is important to request that the installer put an indicator on your
solar system showing that the system is working. It can be as simple as
a small light that comes on when the system is operating. <Top>
Q: What other ways are
there to save on water heating bills?
Besides simply using less hot water, and timing solar-heated water use
to peak when the solar exposure is at its peak, the following strategies
are suggested:
Wash clothes in cold or warm rather than hot water.
Wrap extra insulation around the tank and insulate a few feet of the
hot water line near the tank.
Install an automatic timer so that the heating element comes on only
during certain hours of the day. This may be cost effective if hot water
is needed only for a few hours per day.
Finally, in a one-tank solar system, it's a good idea to have the back-up
element controlled by a timer or manual on/off switch to keep it from
coming on during the day. This strategy allows the solar system to produce
all the hot water without the back-up heater being activated.
Timers may become very attractive if peak-load pricing of electricity
is introduced. In that event, electricity would be priced at a much lower
rate during certain hours of the day than at other times. Peak-load pricing
is being evaluated by many electric power companies. <Top>
Selected references:
Florida Homeowner's Manual for Energy Efficiency, FSEC-EN-20.
FSEC Approved Solar Energy Systems: Domestic Hot Water and Pool
Heating,FSEC-GP-15.
Simplified Sizing Procedure For Solar Domestic Hot Water Systems,FSEC-GP-10.
Solar Domestic Hot Water Systems Repair and Maintenance Manual,
FSEC-GP-34.
Solar Hot Water Energy and Cost Savings for Typical Florida Residential
Installation, FSEC-GP-43
Solar Water and Pool Heating Design and Installation Manual,FSEC-IN-21.
Solar Water Heating Options in Florida, FSEC-EN-9.
Thermal Performance Ratings (FSEC Certified Solar Collectors),
FSEC-GP-14 (DHW) and FSEC-GP-16 (Pool).
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Energy Note:
FSEC-EN-5